Image display device with capacitive energy recovery

ABSTRACT

Device comprising a display panel, preferably organic electroluminescent with passive matrix, comprising an array of columns and an array of rows of electrodes for powering an array of cells and drive means adapted for successively connecting each row electrode to one of the terminals of power supply means of this panel, and during a sequence of connection of a row electrode, for simultaneously connecting one or more column electrodes to the other terminal of the power supply means, and for being able to transfer to each cell to thus be powered the charge of the intrinsic capacitors of the cells linked to the same column electrode as this cell to be powered.

The invention relates to a device for displaying images comprising:

an image display panel comprising a first and a second array ofelectrodes serving an array of electroluminescent cells, where each cellis powered between an electrode of the first array and an electrode ofthe second array.

power supply means linked to said arrays of electrodes,

drive means for each of said cells of the panel, and

means for processing data of the images to be displayed so as toparameterize said drive means.

The first array of electrodes generally corresponds to columns and thesecond array to rows: as power supply means use is generally made of acurrent or voltage generator; the drive means generally comprise columnand row drivers which serve to link the power supply means to the arraysof electrodes.

In such panels, the distance separating the two arrays of electrodes isvery small; at the level of each cell, this distance corresponds to thethickness of an electroluminescent organic layer which is commonly ofthe order of 0.1 μm; therefore, the electrical capacitance between theelectrodes of the two arrays is significant and the intrinsiccapacitance at the level of each cell is therefore high.

Each image to be displayed is divided into pixels, themselves subdividedinto as many subpixels as primary colors; to each subpixel is allocateda luminous intensity datum for the image to be displayed; to display animage, each subpixel of the image is assigned to a cell of the panel.

In such a device, the drive means are adapted:

for successively connecting each electrode of the second array to one ofthe terminals of the power supply means; these steps of the methodcorrespond to the scanning of the lines of the panel;

and, during the sequence of connection of an electrode of the secondarray, for simultaneously connecting electrodes of the first array tothe other terminal of the power supply means.

If the duration of the connection of each electrode of the first arrayor of activation of the column driver depends on the luminous intensitydatum attributed to the cell powered via this column, the duration ofpower supply of a cell corresponds to the width of a voltage or currentpulse, and the driving of the panel is then said to be carried out bypulse width modulation, or is of PWM type.

During the displaying of images, each time a cell of the panel isconnected and powered, its intrinsic capacitor is charged; at the end ofeach sequence of connection of an electrode of the second array or ofthe scanning of a line, all the cells served by this electrode or thisline are disconnected, and before passing to the next sequence ofconnection of another electrode of the second array or of the scanningof another line, all these intrinsic capacitors are discharged so thatthe luminous intensity of the cells served by this other electrode orother line is not disturbed by the intrinsic charges accumulated duringthe previous sequence relating to the previous line.

Accordingly, it is know practice to add an intermediate sequence ofdischarge, for example via shunting means as described in document U.S.Pat. No. 6,339,415—PIONEER; during this intermediate step of discharge,the intrinsic capacitors of the cells of the line that has just beenscanned are discharged to earth.

The drawback of such a procedure of driving with intermediate dischargeof each line is that the capacitive energy of the intrinsic capacitorsis lost.

The document EP 1091340 describes a procedure for capacitive energyrecovery which is limited: specifically, the energy originating from afirst cell is recovered for the benefit of another cell only if thevideo signal to be displayed at this other cell is greater than thevideo signal displayed at the first cell; the drawback of this procedureis that, in the converse case where the video signal is less, thecapacitive energy of the first cell is lost.

The invention is aimed at recovering the capacitive energy in a muchmore complete manner than in the prior art; more precisely, theinvention proposes that the capacitive energy of each cell of a line berecovered so as to reinject it into the cell of the next line on thesame column as a function of the image datum for this cell.

Accordingly, a subject of the invention is a device for displayingimages comprising:

an image display panel comprising a first array and a second array ofelectrodes which serve an array of cells, where each cell is poweredbetween an electrode of the first array and an electrode of the secondarray effecting between them an intrinsic capacitor C_(i),

power supply means for generating a potential difference between twoterminals,

drive means adapted for successively connecting each electrode of thesecond array to one of the terminals of the power supply means, and,during a sequence of connection of an electrode of the second array, forsimultaneously connecting one or more or even all the electrodes of thefirst array to the other terminal of the power supply means,

characterized in that the drive means are adapted for being able, duringeach sequence of connection of an electrode of the second array, totransfer to the cell powered between each electrode of the first arrayand this electrode of the second array, the charge of the intrinsiccapacitors of the other cells linked to the same electrode of the firstarray.

Obviously, if these capacitors are not charged, no transfer of chargecan occur; conversely, in the case where they are charged, this transferof charge may only be partial.

The first array generally corresponds to column electrodes and thesecond array to row electrodes; if we have G rows, there are in generalG cells linked to any given electrode of the first array or column; thecharge which is thus transferred to a cell at the intersection of agiven row and given column, is assumed to have obviously beenaccumulated during a sequence relating to a previous row during whichthe cell at the intersection of this previous row but of the same columnwas connected to the power supply means.

The power supply means of the panel may be a voltage or currentgenerator; they may comprise several generators each assigned to a groupof electrodes.

By virtue of this procedure for driving the panel incorporating means oftransferring capacitive charge from one drive sequence to another of thepanel, a large share of the capacitive energy of the intrinsiccapacitors of the cells of the panel is recovered and the efficiency ofthe display device is substantially improved.

To summarize, a subject of the invention is a device comprising adisplay panel, preferably organic electroluminescent with passivematrix, comprising an array of columns and an array of rows ofelectrodes for powering an array of cells and drive means adapted forsuccessively connecting each row electrode to one of the terminals ofpower supply means of this panel, and during a sequence of connection ofa row electrode, for simultaneously connecting one or more columnelectrodes to the other terminal of the power supply means, and forbeing able to transfer to each cell to thus be powered the charge of theintrinsic capacitors of the cells linked to the same column electrode asthis cell to be powered.

Preferably, these drive means are adapted so that, during each sequenceof connection of an electrode of the second array, the transfer ofcharge via each of the electrodes of the first array is favored at theexpense of the connection of these electrodes to said power supplymeans.

The best profit is thus derived from the charge of the capacitors andthe duration of connection of the cells to the power supply means duringthe displaying of images is thus limited, thereby making it possible tosubstantially improve the efficiency of the device.

Preferably, each image to be displayed being divided into pixels orsubpixels to which are allocated luminous intensity data, each cell ofthe panel being assigned to a pixel or subpixel of the images to bedisplayed, the device comprises means of processing this data so as tobe able, during each sequence of connection of an electrode of thesecond array, to modulate the duration of connection t′_(a1) of eachelectrode of the first array to said power supply means and to modulatethe duration of transfer of charge t′_(a2) of the intrinsic capacitorsof the other cells linked to the same electrode of the first array, as afunction of the luminous intensity datum of the cell powered betweenthis electrode of the first array and this electrode of the secondarray.

Depending on the luminous intensity data to be processed, theseprocessing means will therefore either modulate the duration ofconnection alone, or modulate the duration of charge transfer alone, ormodulate both the duration of connection and the duration of chargetransfer. Preferably, the duration t′_(a2) of charge transfer ismaximized and the duration t′_(a1) of connection is minimized so as tobest improve the efficiency of the device.

It is the duration of connection and/or the duration of transfer whichare therefore modulated as a function of the luminous intensity data;thus, preferably, the display device according to the inventionimplements a pulse width modulation procedure. The control of the panelis therefore performed by modulating the duration of pulses or the widthof electrical signals (“PWM” or Pulse Width Modulation”), as opposed toamplitude modulation (“PAM” or “Pulse Amplitude Modulation”) asdescribed for example in the document EP 1091340 already cited, or inthe document U.S. Pat. No. 6,222,323.

Preferably, the drive means are adapted so that, during each sequence ofconnection of an electrode of the second array, the connection of eachelectrode of the first array to the power supply means is carried out,as appropriate, at the end of a sequence and the transfer of charges iscarried out, as appropriate, at the start of a sequence. In this way,the recovery of capacitive energy is best ensured and is managed in avery simple manner.

Preferably, the device according to the invention is adapted so that:

if t_(L) is the duration of each sequence of connection of an electrodeof the second array,

if C_(i) is the mean value of the intrinsic capacitance of each cell,and if the second array has G electrodes,

if R_(EL) is the mean electrical resistance of an activated cell,

we have: G×C_(i)>40%×0.2 t_(L)/R_(EL).

It is for this type of panel that the capacitive energy then representsmore than 40% on average of the energy consumed for the luminousemission of the cells and that the invention is then of greatestinterest; in practice, the invention is of greatest interest whenG·C_(i)≧10 nF, R_(EL)≧50 kΩ, t_(L)≦500 μs, this generally correspondingto the case of panels having electroluminescent organic cells.

Preferably, the device according to the invention is adapted so that:

if t_(L) is the duration of each sequence of connection of an electrodeof the second array,

if C_(i) is the mean value of the intrinsic capacitance of each cell,and if the second array has G electrodes,

if R_(EL) is the mean electrical resistance of an activated cell,

the ratio t_(L)/R_(EL)·C_(i) is greater than 4.

This condition signifies that the discharge time of the intrinsiccapacitors is much smaller than the line time, thereby allowing fastertransfer and considerable recovery of capacitive energy; this conditionmoreover makes it possible to advantageously simplify the split betweenthe “passive” powering of the cells by charge transfer and thetraditional “active” powering by connection to the terminals of thepower supply means.

Preferably, the cells of the panel are electroluminescent, and eachcomprise an organic electroluminescent layer; preferably, the thicknessof this layer is less than or equal to 0.2 μm; a thickness as small asthis entails high intrinsic capacitances and considerable charges whichit is of particular interest to be able to transfer according to theinvention.

The invention will be better understood on reading the description whichfollows, given by way of nonlimiting example, and with reference to theappended figures in which:

FIG. 1 describes a display device according to an embodiment of theinvention,

FIG. 2 represents a summary diagram of powering an electroluminescentcell of the device of FIG. 1,

FIG. 3 represents the current-voltage characteristic of anelectroluminescent diode corresponding to the cell of FIG. 2,

FIG. 4 represents the discharging of the intrinsic capacitance of thecell of FIG. 2, and the increment in charge corresponding to a time stepof the analog/digital converter of the processing means of the device ofFIG. 1,

FIG. 5 represents the recovery of the capacitive energy for the benefitof a cell of the device of FIG. 1 which is thereafter actively poweredso as to supplement the charge required, without the recovery period andthe active power supply period overlapping,

FIG. 6 represents the partial and adapted recovery of the capacitiveenergy for the benefit of a cell of the device of FIG. 1 which is notthereafter actively powered,

FIG. 7 represents the partial recovery of the capacitive energy for thebenefit of a cell of the device of FIG. 1 which is thereafter activelypowered so as to supplement the charge required, in the case where therecovery period and the active power supply period overlap.

The figures representing time charts take no account of any scale ofvalues so as to better depict certain details which would not be clearlyapparent if the proportions were complied with.

With reference to FIG. 1, the display device according to the inventioncomprises:

an image display panel 1 comprising an array X of anodes X₁, X₂, X₃, X₄. . . arranged in columns and an array Y of cathodes arranged in rowsY₁, Y₂, Y₃, Y₄ . . . serving a two-dimensional array ofelectroluminescent cells 11, where each cell is powered between an anode(column) and a cathode (row).

power supply means 4 comprising on the one hand anodic terminals and onthe other hand cathodic terminals linked to earth (which is notrepresented),

means of driving the cells from this panel comprising a set 2 of columndrivers for controlling the link between the anodes and the anodicterminals, a set 3 of row drivers for controlling the link between thecathodes and the cathodic terminals (here via earth), and means 5 ofdriving these drivers,

means of processing of data of the images to be displayed.

With reference to FIG. 2, the row drivers 3 comprise two positions: aso-called activation position c1, of connection to earth where thecorresponding row is therefore connected to the power supply means 4 viaearth, and a so-called inactivation position c2 of connection to aninverse voltage generator Vdd; the purpose of this inverse voltagegenerator Vdd is to turn off those electroluminescent diodes of thepanel to which it is connected; the voltage Vdd will therefore be chosento be greater, in absolute value, than the voltage delivered by thepower supply means 4 which are linked to the anodes in columns.

Each cell 11 of the panel comprises an electroluminescent organic layer(not represented) between the anode and the cathode which supply it withpower; as this layer operates as a diode, it is represented by a diodeEL in FIGS. 1 and 2; as represented in these figures, each cellcomprises an intrinsic capacitor C_(i) in parallel with this diode.

With reference to FIG. 2, each column driver 2 comprises threepositions: the so-called activation position a1 where the column isconnected to the power supply means 4 delivering a supply voltage V_(a),the “unearthed” position a2 where the column is therefore “floating” andthe so-called inactivation position a3 where the column is connected toa lower discharge limit generator V_(i); the voltage V_(i) willpreferably be chosen to be slightly less than the threshold voltageV_(th) defined hereinbelow, so that we have: V_(i)=V_(th)−ε; conversely,if V_(i)=0, as will be seen later, the part C_(i)×V_(th) of thecapacitive energy of the intrinsic capacitor of each cell is lost.

FIG. 2 represents a cell 11 in the active position powered by the powersupply means 4 via a column driver 2 in position a1 and a row driverheld in position c1 for the duration of scanning t_(L) of this row; asshown in the figure, the row drivers of the other cells of the samecolumn are in position c2 during this time; beyond this duration t_(L),the row driver which was in position c1 passes to the inactivatedposition c2 while the driver of another row passes from the inactivatedposition c2 to the activated position c1.

If the image data assigned to this cell corresponds to a quantity oflight D_(EL), if I_(EL) is the instantaneous electrical intensity in theelectroluminescent diode EL, D_(EL) is proportional to the quantity ofelectricity Q_(EL) passing through the diode over the duration ofscanning t_(L) of the row of this cell so that we have Q_(EL)=∫I_(EL)dt, integrated over the duration t_(L).

The current-voltage characteristic of an electroluminescent diode isillustrated in FIG. 3; to a first approximation, this curve may berepresented by the equation V_(EL)=V_(th)+R_(EL)×I_(EL), where V_(th)corresponds to a triggering threshold voltage and where R_(EL) is thedynamic resistance of the diode.

The total electrical intensity I_(d) injected into the cell 11 is equalto the sum of the intensity i_(EL) passing through the diode of thiscell and of the intensity i_(c) passing through the set of intrinsiccapacitors in parallel with the same anode as this cell 11, i.e. G×C_(i)if G is the number of rows, so that we have:Q _(EL) =∫I _(EL) dt=∫I _(d) dt−∫I _(c) dt, integrated over the durationt _(L).

As illustrated in FIG. 2, ∫I_(c) dt corresponds to the quantity ofcharges stored in all the intrinsic capacitors N×C_(i) of the cells ofthe same column, between the start and the end of connection of the cell11 to the power supply means; this quantity of charges is equal to thedifference between the final charge at the end of connection Q_(Cf) andthe initial charge at the start of connection Q_(Ci); we haveQ_(Cf)=G·C_(i)·V_(a), if however the time of connection to the powersupply means is greater than the charging time of the capacitor (that isto say if t_(a1)>3τ—see hereinbelow).

Only a part Q_(u) of the charge of the intrinsic capacitors of the cellsof this column can be used to allow the emission of a cell of the nextrow L′ in the same column, since the diode of this cell is turned ononly beyond the threshold voltage V_(th); we therefore have:Q_(u)=G·C_(i)(V_(C)−V_(th)), where V_(C) is the voltage across theterminals of these intrinsic capacitors; at the end of the charging ofthese capacitors, we therefore have Q_(u)=G·C_(i) (V_(a)−V_(th)).

If the column driver passes to the floating position a2, if the rowdriver passes to the inactivated position c2 while the driver of anotherrow passes from position c2 to position c1, the intrinsic capacitorsG·C_(i) discharge into the diode of the same column of this other rowaccording to the equation:V _(C)(t)=V _(th)+(V _(a) −V _(th)) (exp(−(t/R _(EL) ·G·C _(i)))), wheret corresponds to an instant of charge transfer.

The time constant for the kinetics of the discharging of the intrinsiccapacitors or for the transfer of charge to the diode therefore equalsτ=R_(EL)·G·C_(i).

After a duration of 1τ, the intrinsic capacitors are discharged to 65%;after a duration of 2τ, the intrinsic capacitors are discharged to 85%;after a duration of 3τ, the intrinsic capacitors are discharged to 95%.

The display device here comprises a data table (“Look Up Table” or LUT)which lists the total charge transferred Q_(t)(t_(t))=∫₀ ^(t) Ci·Vc(t)at each instant of transfer t_(t) from the start of discharge.

At each scan of a row, the means of processing of data of the images tobe displayed are adapted as specified hereinafter to deduce thedurations of setting of each of the column drivers to position a1, a2 ora3, as a function of the luminous intensity data of the pixels orsubpixels corresponding to the cells of this row.

The modulation of the luminous intensity emitted by each cell of thepanel is here of the “PWM” type; the duration t_(c) for which the columndriver remains in the activated position a1 therefore depends on theluminous intensity datum D_(EL) attributed to the cell 11; for thisduration t_(c), the electrical intensity in the cell is programmed toattain a constant value I_(p); in practice, t_(c) corresponds to amultiple of an elementary increment of duration t_(e) which correspondsto the step size of the analog/digital converter used to code theluminous intensity datum D_(EL) as a duration of connection; the valueQ_(e)=I_(p)·t_(e) is called the elementary increment of charge.

A 6-bit converter is for example used, so that t_(L) is divided into 64increments of duration t_(e) and that t_(c)=N·t_(e) where 0≦N≦64.

At the end of a row scan, the part of charge Q_(u) usable to supply adiode with power on the scanning of the next row therefore correspondsto a maximum number of transferable bits N_(a)=Q_(u)/Q_(e).

FIG. 4 illustrates a comparison of the useful charge Q_(u) of theintrinsic capacitor and of the charge increment Q_(e).

If the image datum assigned to the cell of the next row in the samecolumn corresponds to a quantity of light D′_(EL) and to a quantity ofelectricity Q′_(EL) which has to pass through the diode of this cell, wehave:

Q′_(EL)=Q′_(a)+Q_(t) where Q′_(a) is the quantity of electricitypossibly provided by the power supply means 4 for the duration t′_(a1)of connection to the power supply means as a supplement to the quantityof electricity transferred of the connection time of the previous rowQ_(t), originating from the discharging of the intrinsic capacitors ofthe cells of the same column.

Two cases may be distinguished:

either Q_(u)≦Q′_(EL), that is to say the quantity of electricity Q′_(EL)required in the diode exceeds the usable charge of the previous row; wethen have Q′_(a)≧0; the quantities of electricity passing through thediode are then split in accordance with FIG. 5 between a duration ofpassive powering which corresponds to the discharging Q_(t1) of theintrinsic capacitors of the connection time of the previous row and aduration t′_(a1) of flow of the power supply 4; during the passivepowering, the column driver is in the floating position a2; during theactive powering, the column driver is in the active position a1;

or Q_(u)>Q′_(EL), that is to say the usable charge of the previous rowexceeds the quantity of electricity Q′_(EL) required in the diode; wethen have Q′_(a)=0; with reference to FIG. 6, the column driver is inthe floating position a2 for a duration t′_(a2) until the intrinsiccapacitors of the connection time of the previous row discharge by avalue Q_(t2)=Q′_(EL), the residual charge Q_(r)=Q_(u)−Q′_(EL) beingdissipated toward earth via the column driver which for this purpose isset to the deactivated position c3.

The manner in which the means for processing data of images are adaptedfor deducting the durations for which each of the column drivers is setto position a1, a2 or a3 as a function of the luminous intensity data ofthe pixels or subpixels corresponding to the cells of the activated rowwill now be described.

These means are adapted for transmitting to each column driver:

the value “true” or “false” of the inequality Q_(u)≦Q′_(EL),

if this inequality is “true” (case 1), the number N′_(a1) of incrementsof duration t_(e) is such that t′_(a1)=N′_(a1)·t_(e);

if this inequality is “false” (case 2), the number N′_(a2) of incrementsof duration t_(e) is such that t′_(a2)=N′_(a2)·t_(e).

The durations t′_(a1) and t′_(a2) are the durations for which the columndriver of the cell is held respectively in position a1 and in positiona2.

In case 1 where Q_(u)≦Q′_(EL), we calculate N′_(a1) as follows:

We calculate the parameter N′_(a)=(Q′_(EL)−Q_(u))/Q_(e);

If N′_(a)·t_(e)+3τ≦t′_(L) as illustrated in FIG. 5, then there is nooverlap between the duration of passive power supply by transfer ofcharge of the connection time of the previous row and the durationt′_(a1) of active power supply, and N′_(a1)=N′_(a); the charge actuallytransferred Q′_(t) will then be equal to Q_(u); the column driver isthen held in position a2 for a duration t_(L)−N′_(a1)·t_(e), then inposition a1 for a duration N′_(a1)·t_(e); it is not therefore necessaryfor the driver to pass through the position a3.

If N′_(a)·t_(e)+3τ>t′_(L) as illustrated in FIG. 7, then there is anoverlap between the duration of passive power supply t′_(a2) of the celland the duration of active power supply t′_(a1); the charge actuallytransferred Q′_(t) will then be less than Q_(u); specifically, thecharge transfer will be limited by the time t′_(L)−N′_(a1)·t_(e)<3τ.

By using the data table (LUT) described previously, it is possible toascertain the charge transferred at each instant of transfer t_(t) fromthe start of discharge, that is to say Q′_(t)−f(t_(t)).

We thus look for the transfer time t′_(a2) such that Q′_(EL)=f(t′_(a2))+Q_(e)(t′_(L)−t′_(a2))/t_(e) and from this we deduceN′_(a1)=(t′_(L)−t_(a2))/t_(e).

The column driver is then held in position a2 for a duration t′_(a2),then in position a1 for a duration t′_(a1)=N′_(a1)·t_(e)=t′_(L)−t′_(a2).

In case 2 where Q_(u)>Q′_(EL) illustrated by FIG. 6, we calculateN′_(a2) as follows:

Using the data table (LUT) described previously, it is possible toascertain the charge transferred at each instant of transfer t_(t) fromthe start of discharge, that is to say Q′_(t)−f(t_(t)).

We then look for the transfer time t_(a2) such that Q′_(EL)=f(t′_(a2)).

We deduce N′_(a2)=t′_(a2)/t_(e).

The column driver is then held in position a2 for a duration t_(a2),then in position a3 for the duration t′_(L)−t_(a2).

In the scheme for driving the panel just described, the charging time ofthe intrinsic capacitors was considered to be appreciably less than thedischarge time τ=R_(EL)·G·C_(i), for each column of the panel;specifically, the charging time=R_(GEN)·G·C_(i), where R_(GEN) is theinternal resistance of the power supply means 4, to which should beadded here the self resistance of a column electrode which is no longernegligible compared with this internal resistance; as R_(GEN) generallyequals from 1 to 5 kΩ and is much less than R_(EL) (67 kΩ in the examplehereinbelow), the charging time of the intrinsic capacitors is actuallyappreciably less than the discharge time of these capacitors.

We have therefore seen how the image data processing means make itpossible to deduce the durations for which each of the column drivers isset to position a1, a2 or a3 as a function of the luminous intensitydata of the pixels or subpixels corresponding to the cells of anactivated row L′, and as a function of the usable charge Q_(u)originating from the previous row L.

Thus, during each sequence of connection of a row electrode, theduration of connection t′_(a1) of each column electrode and/or theduration of charge transfer t′_(a2) via said column electrode are/ismodulated as a function of the luminous intensity datum of the cellpowered between this electrode of the first array and this electrode ofthe second array. More precisely, it may be seen that, during eachsequence of connection of a row electrode, the connection of each columnelectrode to the power supply means is carried out, as appropriate, atthe end of the sequence for the duration t′_(a1) and the transfer ofcharge is carried out, as appropriate, at the start of the sequence.

By virtue of this procedure for driving the panel, a larger share of thecapacitive energy of the intrinsic capacitors of the cells of the panelis recovered than in the prior art, the recovery of capacitive energy ismanaged in a very simple manner, and the efficiency of the displaydevice is more substantially improved.

The embodiment just described relates therefore to passive panels ofOLED type; this embodiment is applicable in particular to color screenscomprising around G=50 lines, where each cell or subpixel exhibits asize of 100 μm×300 μm and where, by way of indication: V_(th) thresholdvoltage of OLED: 4 V Current density for emission at 0.4 mA/cm² mean 100cd/m²: Line current density on 0.4 × 50: 200 mA/cm² OLED operatingvoltage at 200 8 V mA/cm² OLED mean resistance per unit 20 Ω/cm² area (4V − I_(EL) = 200 mA): → R_(EL): dynamic resistance of (20/0.03 × 0.01) =67 kΩ a diode: Intrinsic capacitance per cm² 56 nF/cm² of panel: → G ·C_(i) then equals: (56 × 0.01 × 0.03 × 50) = 0.84 nF → τ = R_(EL) · G ·C_(i) then equals 56 μs

If the time of an image frame is 20 ms, the activation time t_(L) ofeach line then equals 20 ms/50=0.4 ms.

With the aid of these values, we can evaluate the mean capacitive energywhich could be recovered with regard to the electrical energy dissipatedin the electroluminescent organic diodes, if it is considered that onaverage, over a video sequence to be displayed, only 20% of the diodesare lit:

the quantity of electricity necessary for the charging of a column ofthe panel is 4 V×0.84 nF=3.36 nC,

the quantity of electricity G. Q_(EL) required for the powering of acell of the same column of the panel for 20% of the time of a connectiontime t_(L)=400 μs of a line equals: 4 V×0.2×400 μs/67 kΩ=4.776 nC.

In the absence of capacitive energy recovery, a cell of the panel wouldtherefore consume 8.136 nC; even though the invention allows therecovery of only a share of this capacitive energy, one doesadvantageously manage to decrease the consumption of the panel by 25%.

The invention is of significant interest once the capacitive energyrepresents more than 40% of the energy consumed by a diode, hence onceG×C_(i)>40%×0.2 t_(L)/R_(EL).

Moreover, it is noted that the ratio t_(L)/τ equals 7.15; it istherefore seen that the discharge time 3τ=168 μs is appreciably lessthan the row activation time t_(L)=400 μs, thereby making it possiblehere to recover a very considerable share of the capacitive energy; toobtain a recovery, it is in practice important for the ratiot_(L)/R_(EL)·C_(i) to be greater than 4.

The embodiment as described presents the case where the instant of theend of connection of the cells to the power supply means (column driverin position a1) corresponds to the instant of the end of connection ofthe active row (row driver in position c1); the invention applies alsoto cases where this instant of the end of position a1 of the columndriver precedes the instant of the end of position c1 of the row driver,provided that the values of t′_(a1) and t′_(a2) so permit.

The embodiment just described presents the case where the modulation ofintensity of emission of the cells is carried out by pulse widthmodulation; the invention applies also to display devices employingpulse amplitude modulation.

The invention applies also to panels whose electroluminescent layers arenot organic.

1. A device for displaying images comprising: an image display panelcomprising a first array and a second array of electrodes which serve anarray of cells, where each cell is powered between an electrode of thefirst array and an electrode of the second array effecting between theman intrinsic capacitor C_(i), power supply means for generating apotential difference between two terminals, drive means adapted forsuccessively connecting each electrode of the second array to one of theterminals of the power supply means, and, during a sequence ofconnection of an electrode of the second array, for simultaneouslyconnecting one or more or even all the electrodes of the first array tothe other terminal of the power supply means, wherein the drive meansare adapted for being able, during each sequence of connection of anelectrode of the second array, to transfer to the cell powered betweeneach electrode of the first array and this electrode of the secondarray, the charge of the intrinsic capacitors of the other cells linkedto the same electrode of the first array.
 2. The device as claimed inclaim 1, wherein the drive means are adapted so that, during eachsequence of connection of an electrode of the second array, the transferof charge via each of the electrodes of the first array is favored atthe expense of the connection of these electrodes to said power supplymeans.
 3. The device as claimed in claim 1, wherein each image to bedisplayed being divided into pixels or subpixels to which are allocatedluminous intensity data, each cell of the panel being assigned to apixel or subpixel of the images to be displayed, it comprises means ofprocessing said data so as to be able, during each sequence ofconnection of an electrode of the second array, to modulate the durationof connection t′_(a1) of each electrode of the first array to said powersupply means and to modulate the duration of transfer of charge t′_(a2)of the intrinsic capacitors of the other cells linked to the sameelectrode of the first array, as a function of the luminous intensitydatum of the cell powered between this electrode of the first array andthis electrode of the second array.
 4. The device as claimed in claim 3,wherein the drive means are adapted so that, during each sequence ofconnection of an electrode of the second array, said connection of eachelectrode of the first array to said power supply means is carried out,as appropriate, at the end of a sequence and said transfer of charges iscarried out, as appropriate, at the start of a sequence.
 5. The deviceas claimed in claim 1, wherein it is adapted so that: if t_(L) is theduration of each sequence of connection of an electrode of the secondarray, if C_(i) is the mean value of the intrinsic capacitance of eachcell, and if the second array has G electrodes, if R_(EL) is the meanelectrical resistance of an activated cell, we have: G×C_(i)>40%×0.2t_(L)/R_(EL).
 6. The device as claimed in claim 1, wherein it is adaptedso that: if t_(L) is the duration of each sequence of connection of anelectrode of the second array, if C_(i) is the mean value of theintrinsic capacitance of each cell, and if the second array has Gelectrodes, if R_(EL) is the mean electrical resistance of an activatedcell, the ratio t_(L)/R_(EL)·C_(i) is greater than
 4. 7. The device asclaimed in claim 1, wherein said cells are electroluminescent.
 8. Thedevice as claimed in claim 7, wherein each cell comprises an organicelectroluminescent layer.
 9. The device as claimed in claim 8, whereinthe thickness of said layer is less than or equal to 0.2 μm.